The present invention is a method for sealing large aperture optical windows onto refractory metal tubing and an improved high temperature, lead vapor Raman cell in which a sapphire window is sealed onto a metal tube.
Various laser communication systems for both military and civilian applications have been proposed in which the laser beam is transmitted through space, the atmosphere, or through water. For example, it has been suggested that blue/green submarine communication can be effected through the use of a lead vapor Raman shifted XeCl laser. However, one of the technological problems resides in the high temperature Raman cell construction. The desired lead vapor pressure is about 100 torr, which corresponds to an equilibrium temperature of 1417.degree. C.; however, a useful device might be obtained at 20 torr, corresponding to a temperature of about 1200.degree. C. These temperatures are above the useful range for quartz containers, and therefore a non-equilibrium lead vapor cell using cooled quartz windows and a hot zone based on heat pipe technology was utilized to demonstrate the viability of the approach. However, there are distinct disadvantages in the use of non-equilibrium lead vapor cells. Long life and reliability are unlikely to be achieved and the turbulence created by flowing gases and thermal gradients tends to degrade the beam quality. Metal vapor lasers and metal vapor Raman cells for laser wavelength conversion require heating the metal to very high temperatures in order to get sufficient vapor pressures. Some metals do not have sufficient vapor pressure below 1100.degree. C. which is above the softening point of quartz. Therefore, a sapphire window sealed onto a refractory metal tube is preferably employed.
In order to successfully seal a sapphire window onto a refractory metal tube certain compromises between a number of different factors based on material properties must be considered. These factors include thermal expansion coefficients, metal ductility and sapphire and sealing frit strength under tensile and compressive forces.
Several techniques have been suggested whereby a window is sealed to a tube to form an optical cell for a gas laser or the like. For example, U.S. Pat. No. 3,420,603 discloses a technique where an abrupt change of thickness between the central portion of the window and the outer portion which is sealed to the tube or bulb provides a stress barrier to prevent distortion of the central portion of the window. U.S. Pat. No. 3,183,937 discloses an optical maser disc mounted to a tube end section by means of inserting the tube into a disc aperture which is provided with a trough. FIG. 1 is illustrative of the conventional sealed geometry utilized in mounting a sapphire window onto a tube wall. The conventional technique for sealing a sapphire window onto a metal tube was to use a slightly oversized tube as at 11, positioned the sapphire window 13 inside the tube wall as at 15 and disposed sealing material 17 therebetween. Because the thermal expansion of the metal tube is smaller than the thermal expansion of the sapphire window, the sealing material during cooling is in tension. The resulting stress, which increases with window diameter, can cause hair line cracks of the seal or the seal-metal interface. U.S. Pat. No. 3,420,603, discussed above, is directed to a technique for alleviating stress at this interface.
It is therefore an object of the present invention to provide a technique whereby a metal vapor Raman cell can be provided with a large aperture hermetically sealed optical window for high temperature applications.
It is another object of this invention to provide a large aperture, very high temperature, hermetically sealed optical window equipped lead vapor Raman cell for laser wavelength conversion.